Charged particle beam systems, which are devices where an electron beam is brought to a final focus using magnetic or electrostatic lenses, have their performance limited at this time by spherical or third-order aberration of the lenses. Examples of such electron beam devices are electron microscopes, electron microanalyzers, ion microscopes, and scanning transmission electron microscopes. Spherical aberration occurs in systems where only an image point on the optical axis is desired and where off-axis focusing of the beam occurs.
In 1936, Schertzer (Z. Physik, 101 (1936), page 593) showed that the coefficient of spherical aberration of such lenses always has the same sign and one, therefore, presumes that it cannot be made zero. By stating that the coefficient of spherical aberration always has the same sign, this means that the focused electron beam will cross the optical axis before or in the image plane. According to Schertzer, the only examples of systems having zero aberration are hypothetical and, to date, impractical. (See W. Glase, Z. Physik, 116 (1940), pages 19-33, and A. V. Crewe, Ultramicroscopy, 2 (1977), page 281.) Schertzer's conclusions are based upon the symmetry of the lenses and therefore one might hope to avoid the problem by using systems of lenses which are not cylindrically symmetric. Schertzer in 1947 proposed a scheme which uses what are now called multipole lenses. Specifically, he proposed quadrupoles and octupoles to act together as a device to provide an equal but opposite effect to the aberrations of a round lens. Attempts to build such multipole lenses have been made but have not been successful. These attempts are summarized in the Proceedings of the Ninth International Congress on Electron Microscopy by H. Koops, Toronto, Canada, Vol. 3 (1978), page 185. Failure of these attempts was not because they were based on faulty principles but because the combination of multipole lenses must be made and installed with a degree of precision which is beyond the state of the art. For example, such a device has been constructed at the University of Chicago under direction of one of the coinventors herein, A. V. Crewe. The University of Chicago device has four quadrupoles and three octupoles for a total of 40 pole pieces. The mechanical tolerances were calculated to be such that each pole piece must be positioned to an accuracy of 0.00001 cm. The simplest proposed device uses two sextupoles and one additional round lens which would reduce the number of elements from seven to three.
It is therefore an object of this invention to provide a means for compensating for spherical aberration in a charged particle beam system.
Another object of this invention is to provide a one element means for compensating for spherical aberration in a charged particle beam system.